There a number of big differences between the two.
the My Book Live uses latest gen Power PC processor to give you fastest in class read/write speeds. You will want to make sure your network can support this. What I mean by that is similar to buying a Porsche, then running it on regular gas and only driving it at 35MPH. You want to have a Gigabit router with Wireless N at a minimum. Your PC’s should also have Wireless N. If you don’t, then you won’t see the performance that you can get with the My Book Live.
The MBL also has a new featre called Safepoints - allowing you to backup the complete My Book Live to another NAS on your network.
The MBL also has the WD 2go support - for both PCs/Macs through WD2go.com and with Mobile Apps for iOS and Android.
There are other minor features that make it a great investment, but those are the big ones. Go browse the MBL forums on this web site to get an idea on the product.